The present invention relates to an apparatus for treating polluted water, at least comprising a supply system for water to be treated; an activated-sludge reactor; one or more filter membrane modules; an air supply system and a discharge system for permeate passed by the membranes of the modules.
An apparatus of this type is known from the European Application EP-A-0 510 328. The apparatus described herein comprises a treatment tank, the activated-sludge reactor, in which a number of filter membrane modules are suspended parallel to one another with a certain spacing. The filter membrane modules are preferably constructed in the form of porous flat filter plates, the outermost walls being formed by the membrane. The filter membrane modules are suspended in a housing which is open both at the top and the bottom. The water to be treated is filtered through the membranes by making use of a vacuum pump on the filtrate side in order for the water, which passes the membranes from the reactor and arrives in the porous space of the filter plates, to be drawn off as clean water. In order to prevent undesirable deposition and accretion of layers on the outside of the membranes, provision is made of an air supply system which blows air in the upward direction. Since said air supply is a very critical factor and both too low and too high an air supply rate result in deposition and accretion of layers on the outside of the membrane, provision is additionally made in the said application for air distribution means. The aim is to achieve, with the aid of said means, a uniform distribution of the air stream across the membranes, as a result of which deposition of solid particles is largely prevented.
The abovementioned apparatus does have the drawback, however, that even if the said air distribution means are used, the flow coefficient of the water to be treated and the air supplied is not readily controllable. It is generally found that the mass transfer coefficient between membrane plate and liquid is relatively low, which means rapid deposition and accretion of fouling layers on the membrane. Besides, the apparatus according to EP-A-0 510 328 has the drawback that the water to be treated has to be drawn through and from the filter membrane modules by means of a pump. A further important drawback is that the periodical cleaning required of the membrane plates requires these to be removed from the reactor.